The fabrication of highly integrated electrical circuits with small structural dimensions requires special structuring procedures. One of the most common procedures is the so-called lithographic structuring method. This method comprises depositing a thin layer of a radiation sensitive photoresist on the surface of a semiconductor substrate disk, also referred to as wafer, applying a so-called soft bake step during which the solvent content of the photoresist is reduced and exposing the photoresist to radiation transmitted through a lithographic mask. In the so-called photolithography, electromagnetic radiation is used. During the exposure step, lithographic structures located on the mask are imaged on the photoresist layer. Afterwards, a so-called post exposure bake (PEB) follows, and subsequently a development process is conducted during which structuring of the photoresist layer takes place. The thus structured photoresist layer can be directly used in a subsequent etch process or a doping implant performed in order to form electronic structures in the surface of the wafer.
One main demand of the semiconductor industry is the continuous power enhancement provided by increasingly faster integrated circuits which is interrelated to a miniaturization of the electronic structures. In order to realize smaller dimensions of electronic structures, in particular smaller lateral dimensions also referred to as critical dimensions (CD), lithography facilities and tools are continually improved. In addition, newly developed photoresist materials are applied as well as the performance of existing photoresists is steadily enhanced.
Generally computer simulations reflecting the complex kinetics of chemical reactions proceeding in a volume of a photoresist during different process steps of a lithography procedure are performed in order to predict the performance of novel photoresists and to enhance the material composition of existing photoresists. In particular simulation methods reflecting the chemical kinetics during a development process are considered to have high significance as these methods allow determining an edge profile of a developed photoresist. On the basis of such a simulated resist profile, hindrances limiting lithography performance, in particular surface or profile roughnesses like the so-called line edge roughness (LER) can be detected. As the dimensions of electronic structures are continually decreased, such roughnesses have an ever significant importance.
At present so-called chemically amplified resists (CAR) play a major role in the semiconductor industry. Such photoresists feature a high sensitivity and are therefore particularly used in the volume production of semiconductor chips. In the case of a positive tone chemically amplified photoresist, the photoresist comprises so-called photo acid generator molecules (PAG) and blocking groups, also referred to as dissolution inhibitors, which are attached to polymers of the photoresist. Thereby the inhibitors prevent dissolution of photoresist polymers in a development process.
In the course of an exposure step, during which the photoresist is exposed to electromagnetic radiation e.g. in the deep UV spectral range, photo acid generator molecules of a chemically amplified photoresist are decomposed while generating acid molecules. During a subsequent post exposure bake step, the photogenerated acid diffuses through the polymer matrix of the photoresist, in this way decomposing dissolution inhibitors attached to the polymers (acid catalyzed decomposition). Thereby new acid molecules are generated in the course of such decomposition reactions which can further diffuse through the volume of the photoresist, thus causing further decomposition reactions of dissolution inhibitors. Due to this feature such photoresists are referred to as chemically amplified photoresists. A consequence of the post exposure bake is that initially blocked polymer sites of photoresist polymers are “deprotected” in and close to radiation exposed regions of the photoresist.
In a subsequent development step the photoresist is exposed to a suitable developer solution, normally an aqueous base. The main development mechanism is the ionization reaction of hydroxide ions with deprotected polymer sites of the polymer resin and the subsequent dissolution of the respective polymers. Thereby the probability for dissolution depends on several factors, e.g. polymer length, developer penetration depth, deprotection ratio and developer concentration. Depending on the amount of deprotection resulting from the preceding post exposure bake the polymers are ionized and change their chemical properties to become hydrophilic instead of hydrophobic, thus getting dissolved in the aqueous developer solution.
In the case of a negative tone chemically amplified photoresist, so-called cross-linker molecules are typically attached to the polymers. Again, during an exposure step photo acid generator molecules generate acid molecules due to radiation induced decomposition reactions. In a subsequent post exposure bake, the photogenerated acid molecules diffuse through the volume of the photoresist and react with the cross-linkers attached to the polymers. In this way a cross-linking between respective polymers takes place (acid catalyzed condensation). Thereby new acid molecules are generated, thus being able to induce further cross-linking reactions. In a subsequent development step, polymers comprising cross-linked polymer sites are protected against dissolution. Consequently cross-linked polymers correspond to polymers of a positive tone photoresist which comprise dissolution inhibitors.
As a consequence, the above described performance of a negative tone chemically amplified photoresist can be understood to that effect that initially all polymers do not comprise protecting inhibitor groups, and that inhibitors attached to the polymers and protecting the same against dissolution in a development step are “generated” in radiation exposed regions of the photoresist during a post exposure bake. Though lacking an exact physical or chemical description of the reaction processes occurring in a volume of a negative tone chemically amplified photoresist, this way of understanding will be assumed in the following.
Accordingly, this way of understanding is also applied in the case of a chemically amplified photoresist, at which protection or deprotection of polymers is based on other reaction processes like e.g. a polarity change taking place in the course of a post exposure bake.
At present a number of simulation methods exist which reflect the complex stochastic kinetics of chemical reactions proceeding in a photoresist, in particular a chemically amplified photoresist during a development process. Such methods are e.g. based on the stochastic simulation method described in U.S. Pat. No. 5,446,870. Another example of a method for modeling the dissolution taking place in a positive tone resist during a development step is disclosed in W. D. Hinsberg, F. A. Houle and M. I. Sanchez, Kinetic Model for Positive Tone Resist Dissolution and Roughening, Macromolecules, 8591 (2002).
However, conventional simulation methods generally require a high computing time. As a consequence the practical application of such methods in order to predict an edge profile of a volume of a photoresist after a development process is limited. Moreover the existing methods do not model on a molecular level so that fluctuations like e.g. polymer density variations, fluctuations induced by reaction mechanisms and variations in polymer length and developer penetration depth, which have an impact on roughness of a photoresist like the line edge roughness are taken into account only to some degree. Therefore current methods ability to predict a possible “resist brick wall” where line edge roughness becomes an insurmountable lithography hindrance is shortened.